1. Field of the Invention
This invention generally relates to an ultrasonic diagnostic apparatus for obtaining a tomographic image of an object such as a human body by means of ultrasonic scanning, and more particularly relates to an ultrasonic diagnostic apparatus realizing higher bearing resolution by mechanically moving the array type ultrasonic probe in a direction perpendicular to the array direction along which a plurality of ultrasonic transducer elements are disposed.
2. Description of the Prior Art
An ultrasonic diagnostic method is well known and used to examine an object such as a patient. According to this method, ultrasonic pulses are radiated toward a patient, and ultrasonic pulses reflected by boundaries between body organs which have different acoustic characteristics (acoustic impedance) are received to obtain diagnostic information relating to the organs. Since such an ultrasonic diagnostic method is in non-invasive, a patient being so examined feels substantially no pain. Furthermore, as compared with the conventional X-ray radiation diagnosis, ultrasonic diagnosis causes substantially no trouble of X-ray exposure and easily provides a tomographic image of soft organ tissues. As a result, recently, the ultrasonic diagnostic method has been widely employed.
An actual apparatus for practicing ultrasonic diagnosis is known as an ultrasonic diagnostic apparatus. In order to improve a bearing resolution in a conventional linear scanning type ultrasonic diagnostic apparatus, so-called an electronic focusing method is adopted in which the ultrasonic transducer elements such as piezoelectric elements are sequentially driven by drive signals along the transducer element alignment direction (to be referred to as an "array direction" hereinafter) which have been adjusted by relative delay times, so as to transmit and receive ultrasonic pulses. Furthermore, a focusing method is also known in which the focusing position is sequentially moved along the ultrasonic pulse propagation direction so as to obtain a uniform resolution for any depth in the object. This known method is disclosed, e.g., in U.S. Pat. No. 3,919,683 issued on Nov. 11, 1975.
Meanwhile, a focusing method which uses an acoustic lens is adopted in a direction perpendicular to the array direction (referred to a "lens direction" hereinafter). However, the focal point is fixed according to this type of focusing method using such a lens, so that the resolution of the region spaced apart from the focal point cannot be improved.
The present invention is to eliminate the conventional drawbacks and has for its object to provide an ultrasonic diagnostic apparatus wherein a focal point of an ultrasonic beam can be easily adjusted, and the focal point can be constantly and properly positioned in a vicinity of the region to be examined, thereby obtaining better resolution of a tomographic image.